In the manufacture of integrated circuits, various processes are required to be performed on the silicon wafer that is the substrate of the chips. During manufacture, the wafer is processed from a circular substrate of silicon to a finished wafer containing multiple chips. Wafers are transported in carriers, termed boats or cassettes, to the various operations that are to be performed on the wafer.
Additionally, wafers within a given cassette sometimes need to be sorted as individual wafers as opposed to transporting the entire cassette of wafers. This requirement depends upon which operation is to be next performed on the wafer, or on what the disposition of a given wafer might be.
Any handling of the wafer gives rise to an increased possibility of damage, either by mishandling or by contamination. These factors reduce the yield of the manufacturing process, thereby increasing the cost of production of chips. In a business as competitive as the chip industry, any increase in production cost is highly undesirable.
Therefore, there have been many efforts in the industry to address the problems inherent in handling and sorting wafers. The current state of the art for sorting individual wafers is to use a prealigner to determine the orientation of the wafer, and to then properly position the wafer. Prealigners currently available generally assume that the wafer will be placed on a chuck in a non-centered position. The wafer is spun to determine its eccentricity, and then repositioned in a centered orientation. This method of operation is not only fairly complex, but also requires a two-stage positioning operation. An example of this type of operation is U.S. Pat. No. 5,238,354, issued Aug. 24, 1993 to Volovich for "Semiconductor Object PreAligning Apparatus".
Another method of prealignment is to map the position of the wafer in its non-centered orientation, then use that map to drive an independent positioning means to position the wafer. This allows the user to position the wafer without loading it into the prealigner twice, but does require the independent positioning means. This greatly complicates the positioning operation. This method is exemplified by U.S. Pat. No. 5,308,222, issued May 3, 1994, to Bacchi et al, for "Noncentering Specimen Prealigner".